Cancer is the commonest cause for death among all of the causes for death, and therapies carried out therefor at present are mainly surgical treatment, which may be carried out in combination with radiotherapy and/or chemotherapy. In spite of the developments of new surgical methods and discovery of new anti-cancer agents in recent years, treatment results of cancers have not been improved very much at present except for some cancers. In recent years, by virtue of the development in molecular biology and cancer immunology, cancer antigens recognized by cytotoxic T cells reactive with cancers, as well as the genes encoding the cancer antigens, were identified, and expectations for antigen-specific immunotherapies have been raised.
In immunotherapy, in order to reduce side effects, it is necessary that the peptide or protein to be recognized as the antigen exist hardly in normal cells and exist specifically in cancer cells. In 1991, Boon et al. of Ludwig Institute in Belgium isolated a human melanoma antigen MAGE 1, which is recognized by CD8-positive T cells, by a cDNA-expression cloning method using an autologous cancer cell line and cancer-reactive T cells (Non-patent Document 1). Thereafter, the SEREX (serological identifications of antigens by recombinant expression cloning) method, wherein tumor antigens recognized by antibodies produced in the living body of a cancer patient in response to the cancer of the patient himself are identified by application of a gene expression cloning method, was reported (Patent Document 1, Non-patent Document 2), and several cancer antigens have been isolated by this method. Using a part of the cancer antigens as targets, clinical tests for cancer immunotherapy have started.
On the other hand, as in human, a number of tumors such as mammary gland tumor and squamous cell carcinoma are known in dogs and cats, and they rank high also in the statistics of diseases in dogs and cats. However, at present, no therapeutic agent, prophylactic agent or diagnostic agent exists which is effective for cancers in dogs and cats. Most of tumors in dogs and cats are realized by owners only after they advanced to grow bigger, and in many cases, it is already too late to visit a hospital to receive surgical excision of the tumor or administration of a human drug (an anticancer drug or the like), so that those dogs and cats often die shortly after the treatment. Under such circumstances, if therapeutic agents and prophylactic agents for cancer effective for dogs and cats become available, their uses for canine cancers are expected to be developed.
PDS5A (PDS5, regulator of cohesion maintenance, homolog A) is a protein also called SSC-112 which was identified as a cell cycle regulator involved in distribution of chromosomes, and reported to show higher expression in nasopharyngeal carcinoma, renal cancer, liver cancer and a certain type of breast cancer cells, compared to normal tissues (Patent Document 2, Non-patent Documents 3 to 5). It has been reported that the growth of cancer cells can be suppressed by suppressing expression of PDS5A in cancer cells using an antisense nucleic acid, ribozyme or siRNA against the PDS5A gene or using an antibody that specifically binds to the PDS5A protein, and that cancer cells can be induced to cause apoptosis by administering the full-length PDS5A protein or a partial peptide of the PDS5A protein (Patent Document 3). Further, in Patent Document 3, increase in the mRNA level of the PDS5A protein in cancer cells was confirmed. However, there is no report suggesting that the PDS5A protein and a partial peptide of the protein has an action to induce immunity against cancer cells and hence the protein and a partial peptide of the protein is useful for therapy or prophylaxis of cancer, and whether or not the PDS5A protein has a function as a marker that can be used for diagnosis of cancer has not been confirmed.